Method for producing fiber mat and fiber mat

ABSTRACT

A method for producing a fiber mat that includes: dispersing fibers having a fiber length smaller than a pore diameter of a papermaking wire in a dispersion medium; and matting the dispersed fibers including papermaking the dispersed fibers on a microporous sheet having a pore diameter smaller than that of the papermaking wire and disposed on the papermaking wire.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International application No. PCT/JP2022/014604, filed Mar. 25, 2022, which claims priority to Japanese Patent Application No. 2021-068328, filed Apr. 14, 2021, the entire contents of each of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method for producing a fiber mat containing a liquid crystal polymer, and a fiber mat.

BACKGROUND ART

As a conventional method for producing a fiber sheet (fiber mat), Japanese Patent Application Laid-Open No. 2013-076196 (Patent Document 1) discloses a method for producing a fiber sheet using a papermaking method. Specifically, there is disclosed a method for forming a fiber sheet on a papermaking wire by supplying a fiber suspension in which fibers are dispersed onto the papermaking wire to deposit the fibers on the papermaking wire.

-   Patent Document 1: Japanese Patent Application Laid-Open No.     2013-076196

SUMMARY OF THE INVENTION

In recent years, a fiber sheet such as a nonwoven fabric is used as a filtration filter, an adsorbing material, a heat insulating material, or the like and it is also used as a material for a printed circuit board when combined with an epoxy resin, and the fiber sheet has a wide variety of applications.

The finer the fibers constituting the fiber sheet, the fiber sheet can be made thinner and the variation in thickness can be reduced. In addition, when fine fibers are used in the fiber sheet, filter performance is improved because of an increase the specific surface area which allows the collection of a finer material by a reduced pore diameter.

In addition to the papermaking method, a coater method is used to produce a fiber sheet, but in the case of producing a fiber sheet using fine fibers, the amount of solvent required to wet the fibers increases as the fibers become thinner and the specific surface area thereof increases. For this reason, a solvent recovery type method such as a papermaking method is advantageous in terms of cost.

However, when a papermaking method is used, it is difficult for known papermaking wires to be able to collect fine fibers having a fiber length of 100 μm or less. In order to collect these fine fibers, a method of aggregating the fibers is also conceivable, but a fiber mat formed by papermaking from the aggregated fibers has a poor formation.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a fiber mat containing fine fibers and having a good formation, and a fiber mat.

A method for producing a fiber mat according to the present disclosure includes: dispersing fibers having a fiber length smaller than a pore diameter of a papermaking wire in a dispersion medium; and matting the dispersed fibers including papermaking the dispersed fibers on a microporous sheet having a pore diameter smaller than that of the papermaking wire and disposed on the papermaking wire.

In the method for producing a fiber mat according to the present disclosure, the fine fibers may be at least a part of a liquid crystal polymer powder.

In the method for producing a fiber mat according to the present disclosure, it is preferable that the fibers of the liquid crystal polymer powder have an aspect ratio of the fiber length to a fiber diameter of 10 times or more and 500 times or less, and the fibers of the liquid crystal polymer powder have an average diameter of 2 μm or less.

In the method for producing a fiber mat according to the present disclosure, the method may further include peeling the microporous sheet from the papermaking wire after the matting of the disbursed fibers on the microporous sheet.

In the method for producing a fiber mat of the present disclosure, the microporous sheet may be a woven fabric mesh having a pore diameter of 50 μm or less.

In the method for producing a fiber mat of the present disclosure, the microporous sheet may be a wet nonwoven fabric.

The fiber mat of the present disclosure is constituted of fine fibers and has a formation index of 100 or less as measured by a 3D sheet analyzer.

In the fiber mat of the present disclosure, the formation index may be 10 or more.

In the fiber mat of the present disclosure, the fine fibers may be at least a part of a liquid crystal polymer powder.

According to the present invention, it is possible to provide a method for producing a fiber mat containing fine fibers and having a good formation, and a fiber mat.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a flowchart showing a step for producing a fiber mat.

FIG. 2 is a view showing a matting step of matting a liquid crystal polymer powder in the step for producing a fiber mat.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that in the following embodiments, the same or common parts are denoted by the same reference numerals in the drawings, and description thereof will not be repeated.

<Fiber Mat>

A fiber mat according to the present embodiment is constituted of a liquid crystal polymer powder obtained by finely grinding and fiberizing a liquid crystal polymer. The liquid crystal polymer used in the liquid crystal polymer powder is a thermotropic liquid crystal polymer. A molecule of the liquid crystal polymer has a negative thermal expansion coefficient in an axial direction of a molecular axis and a positive thermal expansion coefficient in a radial direction of the molecular axis. The liquid crystal polymer according to the present embodiment does not have an amide bond.

The fiber mat according to the present embodiment has a formation index of 100 or less as measured with a 3D sheet analyzer. The fiber mat may have the formation index of 20 or less. Moreover, the fiber mat has the formation index of 10 or more.

The 3D sheet analyzer is manufactured by M/K Systems Inc., and an index obtained by quantifying the formation using the 3D sheet analyzer is the formation index. Specifically, the formation index is calculated as follows.

1) A fiber mat is wound around a drum, and the fiber mat is irradiated with a light source (white light) installed on a drum shaft while being rotated.

2) The light transmitted through the fiber mat is detected by a photodetector, and the intensity of the transmitted light at 100,000 measurement points is classified into 64 intensity grades.

3) The formation index is obtained by the maximum frequency (Peak value)/the number of grades (Bins value)×1/100.

Peak value is the number of measurement points in a grade in the histogram having 100,000 measurement points, and the Bins value is a value obtained by multiplying the tens place of the frequency having the maximum number of points less than 100 points by 0.1 and adding the result to the number of grades having 100 points or more in the histogram.

The smaller the formation index is, the higher the uniformity and the better the formation of the fiber mat, and in the present embodiment, by having the formation index, the fiber mat has the higher the uniformity and the better the formation.

<Film>

The fiber mat is pressed to be used as a film (more specifically, a liquid crystal polymer film). In the liquid crystal polymer film, a metal foil such as a copper foil may be bonded to at least one surface, or the metal foil may be bonded to both surfaces. In this case, the liquid crystal polymer film according to the present embodiment can be used as one laminated molded product, for example, as FCCL (Flexible Copper Clad Laminates) capable of forming a circuit by a subtract method.

Here, when a film to which a metal foil is bonded is produced as a laminated molded product, in general, in the case of using a fiber mat having poor formation, thickness unevenness occurs in the bonded metal foil. In addition, when both surfaces of the fiber mat to which the metal foil is bonded are pressed by a hard plate in order to forcibly uniformize the thickness unevenness, the raw material flows from a thick portion to a thin portion of the fiber mat. When a material having an orientation property such as a liquid crystal polymer is used as the raw material, the orientation property is disturbed, and for example, the thermal expansion coefficient varies in the plane. As a result, distortion or undulation occurs in the film.

On the other hand, when a film to which a metal foil is bonded is produced using the fiber mat according to the present embodiment, the fiber mat has the formation index as described above, so that the thickness unevenness can be suppressed from occurring in the bonded metal foil. When the metal foil is bonded, as described later, the main orientation direction of the molecules of the liquid crystal polymer is along the in-plane direction of the metal foil, that is, the in-plane direction of the film. This makes it possible to suppress variations in the coefficient of thermal expansion in the plane and to suppress distortion and undulation.

<Method for Producing Fiber Mat>

FIG. 1 is a flowchart showing a step of producing a fiber mat. A method for producing a fiber mat according to the present embodiment will be described with reference to FIG. 1 .

As shown in FIG. 1 , the method for producing a fiber mat according to the present embodiment includes, as a pre-step (S10), a coarsely grinding step (S11), a finely grinding step (S12), a coarse particle removal step (S13), and a fiberizing step (S14) in this order, and further includes, as a post-step (S20) after the pre-step (S10), a dispersion step (S21) and a matting step (S22).

<Pre-Step>

In the coarsely grinding step (S12), which is the first step of the pre-step (S10), first, a molded product of a liquid crystal polymer is prepared as a raw material. Examples of the molded product of the liquid crystal polymer include pelletized liquid crystal polymer uniaxially oriented, a film-shaped liquid crystal polymer biaxially oriented, and a powdery liquid crystal polymer. As the molded product of the liquid crystal polymer, a pelletized or powdery liquid crystal polymer which is less expensive than the film-shaped liquid crystal polymer is preferable, and the pelletized liquid crystal polymer is more preferable from the viewpoint of production cost. In the present embodiment, the molded product of the liquid crystal polymer preferably does not contain a liquid crystal polymer directly molded into a fibrous form by an electrolytic spinning method, a melt blowing method, or the like. However, the molded product of the liquid crystal polymer may contain a liquid crystal polymer processed into a fibrous form by crushing a pelletized liquid crystal polymer or a powdery liquid crystal polymer.

Next, the molded product of the liquid crystal polymer is coarsely ground to obtain a coarsely ground liquid crystal polymer. For example, the molded product of the liquid crystal polymer is coarsely ground with a cutter mill device to obtain a coarsely ground liquid crystal polymer. The size of a particle of the coarsely ground liquid crystal polymer is not particularly limited as long as the particle can be used as a raw material for the finely grinding step described later. A maximum particle diameter of the coarsely ground liquid crystal polymer is, for example, 3 mm or less.

The method of producing a liquid crystal polymer film according to the present embodiment may not necessarily include the coarsely grinding step (S11). For example, if the molded product of the liquid crystal polymer can be used as a raw material for the finely grinding step, the molded product of the liquid crystal polymer may be directly used as the raw material for the finely grinding step.

Subsequently, in the finely grinding step (S12), the coarsely ground liquid crystal polymer as the liquid crystal polymer is ground in a state of being dispersed in liquid nitrogen to obtain a granular finely ground liquid crystal polymer. In the finely grinding step (S12), the coarsely ground liquid crystal polymer dispersed in liquid nitrogen is ground using a medium. The medium is, for example, a bead. In the finely grinding step (S12), it is preferable to use a bead mill having relatively few technical problems from the viewpoint of handling liquid nitrogen. Examples of an apparatus that can be used in the finely grinding step (S12) include “LNM-08” that is a liquid nitrogen bead mill manufactured by AIMEX Co., Ltd.

In the finely grinding step (S12) of the present embodiment, a grinding method in which the liquid crystal polymer is ground in the state of being dispersed in liquid nitrogen is different from a conventional freeze grinding method. Although the conventional freeze grinding method is a method of grinding a ground raw material while pouring liquid nitrogen onto the ground raw material and a grinder main body, most of the liquid nitrogen is vaporized at the time when the ground raw material is ground. That is, in the conventional freeze grinding method, most of the ground raw material is not dispersed in the liquid nitrogen at the time when the ground raw material is ground.

In the conventional freeze grinding method, heat of the ground raw material itself, the heat generated from the grinder, and the heat generated by grinding the ground raw material vaporize liquid nitrogen in an extremely short time. Thus, in the conventional freeze grinding method, the raw material during grinding located inside the grinder has a temperature much higher than −196° C., which is the boiling point of liquid nitrogen. That is, in the conventional freeze grinding method, grinding is performed under the condition that an internal temperature of the grinder is usually about −100° C. or higher and 0° C. or lower. In the conventional freeze grinding method, when liquid nitrogen is supplied as much as possible, the temperature inside the grinder is approximately −150° C. at the lowest temperature.

For this reason, in the conventional freeze grinding method, for example, when a coarsely ground product of a pelletized liquid crystal polymer uniaxially oriented or a pelletized liquid crystal polymer is ground, grinding proceeds along a plane substantially parallel to an axial direction of a molecular axis of the liquid crystal polymer, and therefore, a fibrous liquid crystal polymer having a very large aspect ratio and a fiber diameter much larger than 3 μm is obtained. That is, in a conventional freeze grinding direction, when the coarsely ground product of the pelletized liquid crystal polymer uniaxially oriented or the pelletized liquid crystal polymer is ground, a granular finely ground liquid crystal polymer as used in the present embodiment cannot be obtained.

In the present embodiment, since the ground raw material is ground in the state of being dispersed in liquid nitrogen, the raw material in a further cooled state can be ground as compared with the conventional freeze grinding method. Specifically, the ground raw material can be ground at a temperature lower than −196° C., which is the boiling point of liquid nitrogen. When the ground raw material having a temperature lower than −196° C. is ground, brittle fracture of the ground raw material is repeated, so that the grinding of the raw material proceeds. As a result, for example, when a uniaxially oriented liquid crystal polymer is ground, not only the fracture progresses in the plane substantially parallel to the axial direction of the molecular axis of the liquid crystal polymer, but also the brittle fracture progresses along the plane intersecting the axial direction, so that the granular finely ground liquid crystal polymer can be obtained.

In the finely grinding step (S12), the liquid crystal polymer formed into granules by brittle fracture in liquid nitrogen is continuously subjected to impact with a medium or the like in a brittle state. Thus, in the liquid crystal polymer obtained in the finely grinding step (S12), a plurality of fine cracks are formed from the outer surface to the inside.

The granular finely ground liquid crystal polymer obtained by the finely grinding step (S12) preferably has a D50 of 100 μm or less, more preferably 50 μm or less as measured by a particle size distribution measuring device by a laser diffraction scattering method. This makes it possible to suppress clogging of the granular finely ground liquid crystal polymer with the nozzle in the following fiberizing step.

Next, in the coarse particle removal step (S13), coarse particles are removed from the granular finely ground liquid crystal polymer obtained in the finely grinding step (S12). For example, by sieving the granular finely ground liquid crystal polymer with a mesh, a granular finely ground liquid crystal polymer under a sieve is obtained, and by removing the granular liquid crystal polymer on the sieve, coarse particles contained in the granular finely ground liquid crystal polymer can be removed. The type of mesh may be appropriately selected, and examples of the mesh include a mesh having an opening of 100 μm. The mesh opening can be appropriately changed according to the desired fiber length of the liquid crystal polymer powder. For example, a mesh having an opening of about 5 μm to 50 μm may be used. The method of producing a liquid crystal polymer powder according to the present embodiment may not necessarily include the coarse particle removal step (S13).

Next, in the fiberizing step (S14), the granular liquid crystal polymer is crushed by a wet high-pressure crushing device to obtain a liquid crystal polymer powder. In the fiberizing step (S14), first, the finely ground liquid crystal polymer is dispersed in a dispersion medium for the fiberizing step. In the finely ground liquid crystal polymer to be dispersed, although coarse particles may not be removed, it is preferable that the coarse particles are removed. Examples of the dispersion medium for the fiberizing step include water, ethanol, methanol, isopropyl alcohol, toluene, benzene, xylene, phenol, acetone, methyl ethyl ketone, diethyl ether, dimethyl ether, hexane, and mixtures thereof.

Then, the finely ground liquid crystal polymer in a state of being dispersed in the dispersion medium for the fiberizing step, that is, the slurry-like finely ground liquid crystal polymer is passed through the nozzle in a state of being pressurized at high pressure. By allowing the liquid crystal polymer to pass through the nozzle at a high pressure, a shearing force or collision energy due to high-speed flow in the nozzle acts on the liquid crystal polymer, and the granular finely ground liquid crystal polymer is crushed, so that the fiberization of the liquid crystal polymer proceeds, and the liquid crystal polymer powder that can be used in the post-step can be obtained. A nozzle diameter of the nozzle is preferably as small as possible within a range in which clogging of the finely ground liquid crystal polymer does not occur in the nozzle from the viewpoint of imparting a high shear force or a high collision energy. Since the granular finely ground liquid crystal polymer in the present embodiment has a relatively small particle diameter, the nozzle diameter in the wet high-pressure crushing device used in the fiberizing step can be reduced. The nozzle diameter is, for example, 0.2 mm or less.

In the present embodiment, as described above, a plurality of fine cracks are formed in the granular finely ground liquid crystal polymer powder. Thus, the dispersion medium enters the inside of the finely ground liquid crystal polymer from fine cracks by pressurization in a wet high-pressure crushing device. Then, when the slurry-like finely ground liquid crystal polymer passes through the nozzle and is located under normal pressure, the dispersion medium that has entered the inside of the finely ground liquid crystal polymer expands in a short time. The dispersion medium that has entered the inside of the finely ground liquid crystal polymer expands, whereby fracture progresses from the inside of the finely ground liquid crystal polymer. Thus, fiberization proceeds to the inside of the finely ground liquid crystal polymer, and the molecules of the liquid crystal polymer are separated per domain arranged in one direction. As described above, in the fiberizing step according to the present embodiment, by defibrating the granular finely ground liquid crystal polymer obtained in the finely grinding step in the present embodiment, it is possible to obtain the liquid crystal polymer powder which has a low content of the lump portion and is in the fine fibrous short form as compared with the liquid crystal polymer powder obtained by crushing the granular liquid crystal polymer obtained by the conventional freeze grinding method.

In the fiberizing step (S14) in the present embodiment, the finely ground liquid crystal polymer may be crushed multiple times by a wet high-pressure crushing device to obtain the liquid crystal polymer powder. The number of times of crushing by the wet high-pressure crushing device is preferably small. The number of times of crushing by the wet high-pressure crushing device may be, for example, five times or less.

The obtained liquid crystal polymer powder is used as a raw material in the post-step. Here, the liquid crystal polymer powder as the fine fibers will be described in detail.

The liquid crystal polymer powder includes at least a fiber portion. The fiber portion is a short fibrous particle whose aspect ratio that is a ratio of a length in a longitudinal direction to a fiber diameter is 10 times or more and 500 times or less, and is a particle having an average diameter of 2 μm or less. Such a liquid crystal polymer powder containing a fiber portion in the fine fibrous short form, which has an aspect ratio of 10 times or more and 500 times or less and an average diameter of 2 μm or less, cannot be produced by a conventionally known production method.

For example, the liquid crystal polymer powder containing a fiber portion having an aspect ratio of 10 times or more and 500 times or less cannot be produced only by an electrospinning method which is a method for producing ultrafine continuous long fibers. It is conceivable that liquid crystal polymer ultrafine long fibers of continuous long fibers produced by the electrospinning method are cut after spinning to be formed into short fibers. However, there is a limit to cutting the liquid crystal polymer ultrafine long fibers of the continuous long fibers having an extremely small fiber diameter and an aspect ratio of approximately infinite short. After cutting the liquid crystal polymer ultrafine long fibers of the continuous long fibers produced by the electrospinning method, the liquid crystal polymer ultrafine long fibers have an aspect ratio of more than 500 times.

The value of the average diameter of the fiber portion is an average value of the fiber diameters in a plurality of fibrous particles constituting the fiber portion. As described above, the liquid crystal polymer powder according to the present embodiment contains fine fibrous particles. The fiber diameter can be measured from image data of the fibrous particles obtained when the fibrous particles are observed with a scanning electron microscope.

The aspect ratio of the fiber portion is preferably 300 or less, more preferably 100 or less. The average diameter of the fiber portion is preferably 1 μm or less.

The fiber portion as an aggregation portion in which fibrous particles are aggregated may be contained in the liquid crystal polymer powder. In the fiber portion, the axial direction of the molecules of the liquid crystal polymer constituting the fiber portion and the longitudinal direction of the fiber portion coincide with each other. In the method of producing a fiber mat according to the present embodiment, since the liquid crystal polymer powder is produced through the above-described fiberizing step, the axial direction of the liquid crystal polymer molecule is strongly oriented along the longitudinal direction of the fiber portion due to fracture between a plurality of domains formed by bundling the molecules of the liquid crystal polymer.

The liquid crystal polymer powder preferably contains a substantially unfiberized lump portion in a content of 20% or less. It is more preferable that the liquid crystal polymer powder does not contain a lump portion. The content of the lump portion is evaluated by the number of the lump portion with respect to the number of the aggregation portions contained in the liquid crystal polymer powder. In the present embodiment, an aggregation portion having a maximum height of more than 10 μm when the liquid crystal polymer powder is placed on a flat surface is the lump portion, and an aggregation portion having a maximum height of 10 μm or less is the fiber portion.

The lump portion as an aggregation portion containing lump-shaped particles and aggregated may be contained in the liquid crystal polymer powder. The lump portion is a substantially unfiberized liquid crystal polymer powder. The lump portion may have a flat outer shape.

In the present embodiment, the liquid crystal polymer powder can have a D50 value of, for example, 13 μm or less as measured by particle size measurement using a particle size distribution measuring device by a laser diffraction scattering method.

The liquid crystal polymer powder used as the raw material in the post-step is not limited to the liquid crystal polymer powder produced in the above-described pre-step.

<Post-Step>

Next, the post-step (S20) will be described. In the dispersion step (S21) which is the first step of the post-step (S20), the above-described liquid crystal polymer powder is dispersed in a dispersion medium to be a slurry-like state. Since the above-described liquid crystal polymer powder in the fine fibrous short form is used, the liquid crystal polymer powder can be dispersed in a highly viscous dispersion medium and thus a homogeneous fiber mat can be produced.

Examples of the dispersion medium used in the dispersion step (S21) include water, ethanol, and mixtures thereof. By using such a dispersion medium, the cost of the dispersion medium can be reduced, and the fiber mat can be produced at low cost.

It is considered that the longitudinal direction of the fiber portion in the liquid crystal polymer powder dispersed in the dispersion medium is not oriented in a specific direction in the dispersion medium.

Next, in the matting step (S22), the slurry-like liquid crystal polymer powder is molded into a liquid crystal polymer fiber mat by a papermaking method. In the papermaking method, the dispersion medium used in the dispersion step can be recovered and reused, and a fiber mat can be produced at low cost.

FIG. 2 is a view showing a matting step of matting a liquid crystal polymer powder in a step of producing a fiber mat. Details of the matting step will be described with reference to FIG. 2 .

As illustrated in FIG. 2 , a paper machine 100 is used in the matting step. The paper machine 100 includes a supply roller 15 that supplies a microporous sheet 10, a winding roller (not illustrated) that collects the microporous sheet 10, a papermaking wire 20, conveying rollers 25 and 26, a storage portion 40 that stores a dispersion medium 41 in which the liquid crystal polymer powder is dispersed, and a heating device 50.

The papermaking wire 20 is a papermaking net of about 80 to 100 mesh. That is, the papermaking wire 20 has a pore diameter of about 150 μm to 180 μm. The papermaking wire 20 is conveyed by the conveying rollers 25 and 26 arranged in the conveyance direction. The conveying roller 26 is disposed on the downstream side of the conveying roller25. The papermaking wire 20 is conveyed by the conveying rollers 25 and 26 so as to pass through the storage portion 40.

The supply roller 15 supplies the microporous sheet 10 onto the papermaking wire 20. The microporous sheet 10 disposed on the microporous sheet 10 is conveyed by the papermaking wire 20 so as to pass through the storage portion 40. The microporous sheet 10 having passed through the storage portion 40 is peeled off from the papermaking wire 20 and wound up by a winding roller.

The microporous sheet 10 has a mesh finer than that of the papermaking wire 20. The microporous sheet 10 is preferably about 157 mesh or more. That is, the microporous sheet 10 preferably has a pore diameter of about 100 μm or less. Thus, the fine liquid crystal polymer powder dispersed in the dispersion medium can be collected.

More preferably, the microporous sheet 10 preferably has a pore diameter of about 5 μm to 50 μm. When the pore diameter of the microporous sheet 10 is too small, the water-filterability is deteriorated, and the time required for dehydration becomes long. On the other hand, when the pore diameter of the microporous sheet 10 is too large, fine fibers (fine liquid crystal polymer powder) are hardly collected, and the yield becomes poor.

When the microporous sheet 10 having variations in pore diameter is selected, it affects the formation of the fiber mat to be formed, and therefore when high uniformity is required for the fiber mat, a mesh periodically knitted in a mesh shape is preferable. That is, as the microporous sheet 10, it is preferable to use a mesh having a uniform pore diameter and no bias in the location of pores.

As the microporous sheet 10, for example, a woven fabric mesh having a pore diameter of 50 μm or less can be used. As the woven fabric mesh, for example, a woven fabric mesh constituted of synthetic fibers such as polyester can be adopted.

As the microporous sheet 10, a wet nonwoven fabric may be used. As the wet nonwoven fabric, a wet nonwoven fabric constituted of microfibers can be used. The microfiber is constituted of, for example, a synthetic fiber such as polyester. As the wet nonwoven fabric, a wet nonwoven fabric having a basis weight of 15 g/m² or less may be used.

The heating device 50 is disposed on the downstream side of the storage portion 40 in the conveyance direction. The heating device 50 heats and dries the liquid crystal polymer powder 30 which is subjected to papermaking on the microporous sheet 10.

The matting step (S21) includes a papermaking step, a peeling step, and a drying step. In the matting step (S21), first, the dispersed liquid crystal polymer powder is subjected to papermaking on the microporous sheet 10 in the papermaking step. Specifically, the microporous sheet 10 supplied onto the papermaking wire 20 is conveyed by the papermaking wire 20 and allowed to pass through the storage portion 40. At this time, the liquid crystal polymer powder dispersed in the dispersion medium 41 stored in the storage portion 40 is subjected to papermaking on the microporous sheet 10.

Subsequently, in the peeling step, the microporous sheet obtained by papermaking the dispersed liquid crystal polymer powder thereon is peeled off from the papermaking wire 20. Specifically, the microporous sheet 10 is wound by a winding roller to convey the microporous sheet 10 in a direction different from the direction of the papermaking wire 20. The papermaking wire 20 may be conveyed in a direction different from the direction of the microporous sheet 10 by the conveying roller 26.

Next, in the drying step, the liquid crystal polymer powder 30 which is subjected to papermaking on the microporous sheet 10 is heated and dried by the heating device 50. As a result, a fiber mat constituted of a liquid crystal polymer is formed on the microporous sheet 10.

The microporous sheet 10 on which the fiber mat is formed is wound by the winding roller in the winding step.

As described above, by conveying the fiber mat together with the microporous sheet 10 in a state in which the liquid crystal polymer powder is subjected to papermaking on the microporous sheet 10, the fragile fiber mat in which the entanglement between the fine fibers is weak can be carried to the next step without being damaged.

<Method for Producing Film>

Subsequently, the fiber mat is peeled off from the microporous sheet 10, and the fiber mat is heat-pressed to obtain a liquid crystal polymer film. By the heat-pressing step, the thickness of the liquid crystal polymer film becomes thinner than that of the fiber mat.

In the heat-pressing step, the liquid crystal polymer fiber mat is heat-pressed together with, for example, a copper foil. Thus, the heat-pressing step also serves as a step of bonding the liquid crystal polymer film and the copper foil to each other, so that the liquid crystal polymer film to which the copper foil is bonded can be obtained at low cost. In the case where the liquid crystal polymer fiber mat is heated for a long time in the heat-pressing step, it is preferable that the liquid crystal polymer fiber mat is heated and pressed in a vacuum.

In the heat-pressing step, it is preferable to perform heat-pressing at a temperature lower by about 5° C. to 15° C. than the endothermic peak temperature of the liquid crystal polymer constituting the liquid crystal polymer powder. When heat-pressing is performed at a temperature lower by about 5° C. to 15° C. than the endothermic peak temperature, sintering of the liquid crystal polymers easily proceeds.

In the heat-pressing step, a polyimide film, a PTFE film, or a composite sheet including a reinforcing material such as a glass fiber fabric and a heat-resistant resin may be interposed as a release film between a pressing machine used in the heat-pressing step and the liquid crystal polymer fiber mat. In place of the polyimide film, an additional copper foil may be interposed between the pressing machine and the liquid crystal polymer fiber mat. This makes it possible to obtain a liquid crystal polymer film in which copper foils are bonded to both surfaces. The liquid crystal polymer film in which the copper foils are bonded to both surfaces can be used as a double-sided copper clad FCCL.

By heat-pressing, among the fiber portions of the liquid crystal polymer powder in the fiber mat, the fiber portion having the longitudinal direction in a direction along the thickness direction of the fiber mat is heated while being pushed down in the in-plane direction of the copper foil. Since the liquid crystal polymer constituting the liquid crystal polymer powder has the axial direction of the molecule in the longitudinal direction of the fiber portion, the axial direction of the molecule of the liquid crystal polymer is also pushed down in the in-plane direction of the copper foil.

Thus, in the molded liquid crystal polymer film, the main orientation direction of the molecules of the liquid crystal polymer is along the in-plane direction of the copper foil, that is, the in-plane direction of the liquid crystal polymer film. However, in the lump portion, the axial direction of the molecule is random, and depending on a ratio of the lump portion contained in the liquid crystal polymer film, there is a portion where the axial direction of the molecule of the liquid crystal polymer is directed in the thickness direction of the liquid crystal polymer film.

Specifically, in the in-plane direction of the liquid crystal polymer film, there are a region in which a ratio where the molecules of the liquid crystal polymer face the axial direction in the thickness direction of the liquid crystal polymer film is large and a region in which a ratio where the molecules of the liquid crystal polymer face the in-plane direction is large. More specifically, except for the molecules constituting the lump portion, the axial direction of each molecule constituting the liquid crystal polymer is oriented along the in-plane direction of the liquid crystal polymer film over the thickness direction of the liquid crystal polymer film.

In addition, the liquid crystal polymer powder in the fiber mat may be bonded to each other while the fiber portions are entangled with each other. Thus, the liquid crystal polymer in the liquid crystal polymer film has a structure in which molecules are entangled with each other. Since the fiber portion has a larger surface area than a spherical liquid crystal polymer having the same volume, a bonding area also increases when the liquid crystal polymer powders are bonded to each other by the heat-pressing step. Thus, the liquid crystal polymer film according to the present embodiment is improved in toughness and folding resistance.

If necessary, the metal foil bonded to the liquid crystal polymer film may be removed by etching or the like. As a result, a single liquid crystal polymer film to which the metal foil is not bonded is obtained.

EXAMPLE

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited thereto. In the experimental example, the fiber mats according to Examples 1 and 2 were prepared, and their formation indexes were measured using a D sheet analyzer manufactured by M/K Systems Inc. In Comparative Example 1, an attempt was made to produce a fiber mat, but the fiber mat could not be produced as described later.

Example 1

In Example 1, first, as a liquid crystal polymer molded product as a raw material, a pelletized liquid crystal polymer was coarsely ground by charging the liquid crystal polymer into a cutter mill device. The endothermic peak temperature of the liquid crystal polymer used in Example 1 was 315° C. The coarsely ground film-shaped liquid crystal polymer was discharged from a discharge hole having a diameter of 3 mm provided in a cutter mill device to obtain a coarsely ground liquid crystal polymer.

Next, the coarsely ground liquid crystal polymer was finely ground with a liquid nitrogen bead mill (LNM-08 manufactured by AIMEX Co., Ltd.). In the grinding with the liquid nitrogen bead mill, a vessel capacity was set to 0.8 L, zirconia beads having a diameter of 5 mm were used as media, an amount of the media charged was set to 500 mL, 30 g of the coarsely ground liquid crystal polymer was charged, and grinding treatment was performed at a rotation speed of 2000 rpm for 120 minutes. In the liquid nitrogen bead mill, the coarsely ground liquid crystal polymer is dispersed in liquid nitrogen to perform wet grinding treatment. As described above, the coarsely ground liquid crystal polymer was ground in the liquid nitrogen bead mill to obtain a granular finely ground liquid crystal polymer.

Next, the finely ground liquid crystal polymer was wet-classified with a mesh having an opening of 100 μm to remove coarse particles contained in the finely ground liquid crystal polymer, and the finely ground liquid crystal polymer having passed through the mesh was recovered. In Example 1, a mesh having an opening of 100 μm was used, but a mesh having a smaller mesh opening than the mesh may be used for classification.

Next, the finely ground liquid crystal polymer from which the coarse particles had been removed was dispersed in a 20 wt % ethanol aqueous solution. An ethanol slurry in which the finely ground liquid crystal polymer was dispersed was repeatedly ground five times using a wet high-pressure crushing device under the conditions of a nozzle diameter of 0.2 mm and a pressure of 200 MPa to be formed into fibers. Star Burst HJP-25060 manufactured by Sugino Machine Limited was used as a wet high-pressure crushing device. As a result, a liquid crystal polymer powder dispersed in an ethanol aqueous solution was obtained.

Next, water and ethanol were added in required amounts to prepare 2.2 g of the liquid crystal polymer powder with respect to 30 L of a 50 wt % ethanol aqueous solution, and the slurry-like liquid crystal polymer powder was molded into a fiber mat by a papermaking method. The liquid crystal polymer powder dispersed in a dispersion medium was subjected to papermaking on a microporous sheet of polyester mesh having a pore diameter of 11 μm using a square sheet machine 2555 manufactured by Kumagai Riki Kogyo Co., Ltd. as a paper machine.

Subsequently, the fiber mat was molded on the microporous sheet by heating and drying at a temperature of 100° C. using a hot air dryer. The basis weight of the fiber mat was about 35 g/m².

Two fiber mats according to Example 1 were produced, and the formation index of each of the fiber mats was 100 or less, specifically, 98.4 or 77.2. It was confirmed that the fiber mat according to Example 1 contained fine fibers and had a particularly good formation.

Example 2

In Example 2, a microporous sheet formed of a wet nonwoven fabric of polyester microfibers and having a basis weight of about 35 g/m² was used. A fiber mat was obtained in substantially the same manner as in Example 1 except for the above points.

Two fiber mats according to Example 2 were produced, and the formation index of each of the fiber mats was 10 or more, specifically, 15.6 or 11.7. In Example 2, although the formation was lower than that in Example 1, a fiber mat containing fine fibers and having a good formation was obtained.

Comparative Example 1

In Comparative Example 1, a liquid crystal polymer powder dispersed in a dispersion medium was subjected to papermaking on a papermaking wire without using a microporous sheet. As a papermaking wire, LTT-9FE manufactured by Nippon Filcon Co., Ltd. was used.

In Comparative Example 1, since a microporous sheet was not used, most of the liquid crystal polymer powder passed through the pores of the papermaking wire, and a fiber mat could not be formed.

In the above-described embodiments and examples, the case where the fine fibers are a liquid crystal polymer powder has been described as an example, but the fine fibers are not limited to the liquid crystal polymer powder. As long as the fine fibers have a fiber length smaller than the pore diameter of the papermaking wire 20 as described above, organic fibers containing an organic substance as a main component can be appropriately adopted as the fine fibers.

As described above, the embodiments and examples invented herein are illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, and considered to encompass all of modifications within the spirit and scope equivalent to the claims.

DESCRIPTION OF REFERENCE SYMBOLS

-   -   10: Microporous sheet     -   15: Supply roller     -   20: Papermaking wire     -   25, 26: Conveying roller     -   30: Liquid crystal polymer powder     -   40: Storage portion     -   41: Dispersion medium     -   50: Heating device     -   100: Paper machine 

1. A method for producing a fiber mat, the method comprising: dispersing fibers having a fiber length smaller than a pore diameter of a papermaking wire in a dispersion medium; and matting the dispersed fibers including papermaking the dispersed fibers on a microporous sheet having a pore diameter smaller than that of the papermaking wire and disposed on the papermaking wire.
 2. The method for producing a fiber mat according to claim 1, wherein the fibers are at least a part of a liquid crystal polymer powder.
 3. The method for producing a fiber mat according to claim 2, wherein the fibers of the liquid crystal polymer powder have an aspect ratio of the fiber length to a fiber diameter of 10 times or more and 500 times or less, and the fibers of the liquid crystal polymer powder have an average diameter of 2 μm or less.
 4. The method for producing a fiber mat according to claim 2, wherein the liquid crystal polymer powder is a thermotropic liquid crystal polymer.
 5. The method for producing a fiber mat according to claim 2, wherein a molecule of the liquid crystal polymer powder has a negative thermal expansion coefficient in an axial direction of a molecular axis thereof and a positive thermal expansion coefficient in a radial direction of the molecular axis thereof.
 6. The method for producing a fiber mat according to claim 2, wherein the liquid crystal polymer powder does not have an amide bond.
 7. The method for producing a fiber mat according to claim 1, further comprising peeling the microporous sheet from the papermaking wire after the matting of the disbursed fibers on the microporous sheet.
 8. The method for producing a fiber mat according to claim 1, wherein the microporous sheet is a woven fabric mesh.
 9. The method for producing a fiber mat according to claim 8, wherein the pore diameter of the microporous sheet is 100 μm or less.
 10. The method for producing a fiber mat according to claim 1, wherein the microporous sheet is a wet nonwoven fabric.
 11. The method for producing a fiber mat according to claim 1, wherein the pore diameter of the microporous sheet is 100 μm or less.
 12. The method for producing a fiber mat according to claim 1, wherein the pore diameter of the microporous sheet is 5 μm to 50 μm.
 13. A fiber mat comprising fibers and having a formation index of 100 or less as measured by a 3D sheet analyzer.
 14. The fiber mat according to claim 13, wherein the formation index is 10 or more.
 15. The fiber mat according to claim 13, wherein the fibers are at least a part of a liquid crystal polymer powder.
 16. The fiber mat according to claim 15, wherein the fibers of the liquid crystal polymer powder have an aspect ratio of a fiber length to a fiber diameter of 10 times or more and 500 times or less, and the fibers of the liquid crystal polymer powder have an average diameter of 2 μm or less.
 17. The fiber mat according to claim 15, wherein a main orientation direction of molecules of the liquid crystal polymer powder is along an in-plane direction of the fiber mat.
 18. The fiber mat according to claim 15, wherein the liquid crystal polymer powder is a thermotropic liquid crystal polymer.
 19. The fiber mat according to claim 15, wherein a molecule of the liquid crystal polymer powder has a negative thermal expansion coefficient in an axial direction of a molecular axis thereof and a positive thermal expansion coefficient in a radial direction of the molecular axis thereof. 